1. Field of the Invention
This invention relates to an apparatus for observing activity in the solar atmosphere particularly for observing and measuring solar magnetic fields.
2. The Prior Art
The sun is of seeming quiet brilliance when viewed directly or, e.g., through a dark filter. However the sun when viewed through a filter equipped telescope, reveals a boiling surface, sunspots, sudden brightenings (flares) and ejections of material (sprays). These activities, particularly solar flares and their effects on Earth, are unpredictable, as man, at the present time, has been unable to extract enough information from pre-flare observations. However a promising signal of an imminent major flare appears to be the emerging magnetic flux in the photosphere of the sun. Early detection of such emerging fields might give several hours warning before a major flare onset. That is, the intensity and direction of the magnetic field at the solar surface may be inferred from measurements of the polarization of sunlight in very narrow spectral bands. That effect was discovered in the laboratory by a P. Zeeman in 1896, and it was applied very shortly thereafter by a G. E. Hale to the measurement of magnetic fields in sunspots.
The Zeeman effect is characterized by the splitting of atomic spectral lines into two or more orthogonally polarized components, which splitting occurs when the atoms emitting the light are in a magnetic field. Using the optic system described below, the polarized components are separated and directed in turn through, e.g., two narrow band pass filters on to an image receptor for comparison purposes The process is repeated at different bandpass filter settings to provide a solar magnetic field map as more fully described below. In the past, an optical system called a solar magnetograph has been assembled and aimed at the sun in an attempt to detect such pre-flare emerging magnetic flux. However such prior art magnetograph, because of certain mechanical and design encumbrances has not provided a magnetograph sensitive enough to reliably detect pre-flare magnetic changes.
In particular, such prior art magnetograph has employed in the lens system, an ultra narrow band filter (for spectral discrimination) called a birefringent filter of heavy and complex layered design, requiring mechanical devices for wave length tuning. In addition such filters have proved difficult of repair.
Thus despite a recognized need for improved magnetographs, no new instrument has been built in the preceding twenty-five years. This is because the above birefringent filters, in addition to the above difficulties, have been too expensive, lacked sufficient light throughput and were difficult to tune in wavelength. Often such filters proved unreliable. Many such existing units were down for years, awaiting availability of skilled personnel.
Accordingly there has been a need and market for an improved instrument for measuring solar magnetism that overcomes the above prior art shortcomings.
There has now been discovered an improved solar measuring instrument wherein such a birefringent filter is replaced with a light-weight and durable component which admits such a narrow solar passband as to operate with sufficient sensitivity to monitor the solar atmosphere for magnetic field changes that presage solar flares or other disturbances.